Field of the Invention
The present invention relates to rolling bearings to be used for supporting a main shaft of a machine tool, for example.
Description of Related Art
In main shaft devices of machine tools, temperature increase in the device needs to be suppressed at a small level in order to ensure machining precision. However, in recent machine tools, there is a trend of increasing the speed thereof in order to improve machining efficiency, and the amount of heat generated from bearings supporting the main shaft is on the increase, associated with the increased speed. In addition, there are an increasing number of so-called motor-built-in type devices in which a driving motor is incorporated. Such a configuration also serves as a factor that causes heat to be generated in the device.
Heat generation during bearing rotation causes increase in the contact surface pressure between bearing rings and rolling elements, and thus increases the risk of damage of the bearing. Therefore, when trying to achieve high speed and high precision of the main shaft, temperature increase in the bearing due to heat generation is desired to be suppressed as much as possible. As means for suppressing temperature increase in the bearing, there is a technique of injecting a cooling fluid composed of a mixture of air and oil or composed of only air, to a bearing in operation, thereby cooling the bearing.
If the bearing cooling effect is improved by injecting such a cooling fluid to the bearing in operation, the bearing can operate at a faster speed. However, near the revolving rolling elements, an air curtain is formed by swirl flow that occurs around the axis, and if a large amount of the cooling fluid is injected toward this air curtain, collision sound occurs. This collision sound causes noise. Thus, a configuration is employed in which a nozzle configured to inject the cooling fluid is tilted forward in the rotation direction of the main shaft, thereby trying to reduce such noise (for example, Patent Documents 1 and 2).